It had always been a difficult question what caused the extraordinary luminosity of Aristarchus. No end of hypothesis had been invented to account for it. Now I was to assist in settling these questions forever. From Cape Heraclides to Aristarchus the distance in air line was something over 300 miles. Our course lay across the northeastern part of the Sea of Showers, with enormous cliffs, mountain masses and peaks shining on the right, while in the other direction the view was bounded by the distant range of the lunar Appenines, some of whose towering peaks, when viewed from our immense elevation, appeared as sharp as the Swiss Matterhorn. When we had arrived within about a hundred miles of our destination we found ourselves, floating directly over the so-called Harbinger Mountains. The serrated peaks of Aristarchus then appeared ahead of us, fairly blazing in the sunshine. It seemed as if a gigantic string of diamonds, every one as great as a mountain peak, had been cast down upon the barren surface of the moon and left to waste their brilliance upon the desert air of this abandoned world. As we rapidly approached the dazzling splendor of the mountain became almost unbearable to our eyes, and we were compelled to resort to the devise, practised by all climbers of lofty mountains, where the glare of sunlight on snow surfaces is liable to cause temporary blindness, of protecting our eyes with neutral-tinted glasses. Professor Moissan, the great French chemist and maker of artificial diamonds, fairly danced with delight. "Voila! Voila! Voila!" was all that he could say. When we were comparatively near, the mountain no longer seemed to glow with a uniform radiance, evenly distributed over its entire surface, but now innumerable points of light, all as bright as so many little suns, blazed away at us. It was evident that we had before us a mountain composed of, or at least covered with, crystals. Without stopping to alight on the outer slopes of the great ring-shaped range of peaks which composed Aristarchus, we sailed over their rim and looked down into the interior. Here the splendor of the crystals was greater than on the outer slopes, and the broad floor of the crater, thousands of feet beneath us, shone and sparkled with overwhelming radiance, as if it were an immense bin of diamonds, while a peak in the center flamed like a stupendous tiara incrusted with selected gems. Eager to see what these crystals were, the car was now allowed rapidly to drop into the interior of the crater. With great caution we brought it to rest upon the blazing ground, for the sharp edges of the crystals would certainly have torn the metallic sides of the car if it had come into violent contact with them. Donning our air-tight suits and stepping carefully out upon this wonderful footing we attempted to detach some of the crystals. Many of them were firmly fastened, but a few--some of astonishing size--were readily loosened.